1. Field of the Invention
This invention is generally directed to systems and equipment for recovering contaminants and particularly hydrocarbons from other materials and especially to a system for recovering hydrocarbons from soils. The hydrocarbon contaminants may be in varied forms but are generally gasoline, diesel fuels and oils such as heating and crude oils. The method of the present invention involves introducing the contaminated soil into a thermal desorption unit where it is conveyed and exposed to temperatures which are supplied centrally with respect to at least one annular conveying chamber created by two concentric cylindrical rotors. The material is introduced under vacuum and the material is conveyed under partial vacuum as it is heat treated. The heat of the vapor is maintained relatively low due to the reduced atmosphere within the thermal desorption unit and the rate of conveyance of the material through the unit is varied so as to insure that contaminants are completely vaporized before the material is discharged from the desorption unit. Vapors are withdrawn under vacuum and are separated from fines which are returned to the processing unit while the vapor is condensed and separated thereby recapturing the hydrocarbons for further use.
In the preferred embodiment, the thermal desorption apparatus of the present invention includes two horizontally disposed and concentrically oriented material conveying cylinders which are rotatably mounted with respect to a common support base. The cylinders are mounted about one or more axially disposed heating units which are preferably electrical heaters. Material to be treated is introduced through a vacuum feed system and is introduced at a variable control rate depending upon sensed parameters of the operating system. The material is introduced into an annular conveyance passageway defined between the two rotating cylinders with the rate of conveyance being determined again by operating parameters depending upon the type of material being treated, the contaminants to be removed and the moisture content of the material. The apparatus further provides for the removal and separation of vapors utilizing separators which maintain the conveyance area between the rotating cylinders at a reduced pressure so that the amount of energy necessary to heat the material to vaporize volatile contaminants is significantly reduced. The heat exchanger design insures that all heat generated must pass through the material being treated. A cyclone separator is provided for separating vapors from the conveyance area between the rotating cylinders of the heat exchangers and vapors are also received from an expansion chamber or secondary cyclone separator mounted between first and second sets of heat exchange and material conveyance cylinders.
2. History of the Related Art
Hydrocarbon pollution and contamination of soil areas is an ever growing problem which requires the immediate concern of individuals as well as government and private agencies. The destruction to the environment through accidental spills of oil and other hydrocarbon products damages thousands and possibly millions of acres of shoreline and other onshore areas worldwide each year. Over the years numerous techniques have been proposed for cleaning hydrocarbon spills and recovering hydrocarbon products, however most systems have dealt with recovering bulk liquids without providing an adequate solution for separating oils and other hydrocarbons entrained in the soil. Some methods of recovering hydrocarbons involve the use of solvents which are sprayed or applied across the soil after which the soil is treated with steam to break the hydrocarbons down chemically. Unfortunately, this type of recovery does not adequately separate sufficient hydrocarbons from the soil and often results in allowing hydrocarbons to seep deeper within the soil creating even greater contamination to subsurface soil areas. Further, as a result of steam treatment type procedures, the hydrocarbons may be contained in runoffs which are directed to streams and rivers remote from the contaminated site thereby causing additional ecological damage.
In oil field technology there have been numerous devices developed for separating contaminants and hydrocarbons from cuttings waste both in oil base and water base drilling fluid systems. In such systems, during the process of drilling a well, drilling fluids are utilized to flush away down hole cuttings at the point where the drill bit cuts through the earth. Thereafter, the drilling fluid is conveyed above ground where the drilling fluid is sifted and otherwise treated to remove cuttings and other contaminants from the fluid.
In U.S. Pat. No. 4,313,785 to the inventor of the present application for patent, a method and apparatus is disclosed for removing hydrocarbons which adhere to fragments of rock produced during well drilling operations and, in particular, where an oil base drilling fluid is used during such operations. The method and apparatus involve a two-stage process for burning hydrocarbons and other volatile materials from cuttings wherein each stage of the apparatus are mounted upon a common rotatable shaft for transferring the materials from one stage to the other. The system is designed to utilize some of the waste gas recovered during the treatment process for purposing of providing heat for vaporizing hydrocarbons from the solid material. The apparatus includes at least one burner wherein the contaminated material is introduced and conveyed by means of rotatable augers. The side walls of the burners or heating units have heating elements embedded therein and in some instances other combustion or heated gas may be blown through the units. Within the device, the material is heated as it is conveyed therethrough with the volatile materials being separated as the temperature is raised significantly in a first stage. Thereafter, the solid material is discharged to a second rotating conveyor unit wherein further separation of the less volatile components is accomplished by supplying additional heat. The patent further discusses that the heating and separator unit not only has potential for use in oil field technology but also may be utilized to treat low levels of hydrocarbon contamination such as is present and occurs along beaches and the like where oil spills have accumulated on sand. The unit was therefore designed to be portable to allow a wide range of uses for separating hydrocarbons from various types of soils.
Unfortunately, the previous unit is not sufficiently efficient and requires the generation of excessive energy to heat materials being treated which makes the unit impractical for use in many areas, especially where lower levels of hydrocarbon contamination are encountered. In addition, the system did not provide for adequately regulating the conveyance of materials through the heating unit to insure optimum separation of the volatile components with the rate of conveyance being automatically controlled by monitoring parameters of the materials as they are treated.
Another type of system for extracting contaminants such as hydrocarbons from cutting wastes in oil field well drilling is disclosed in U.S. Pat. No. 4,606,283 to DesOrmeaux et al. This patent describes a system for extracting volatile hydrocarbons from cutting wastes wherein the material to be treated is introduced into hoppers which feed a plurality of screw conveyors which are horizontally mounted on a base unit. The screw conveyors include heating elements disposed in the outer walls which heating elements are utilized to generate sufficient heat to vaporize the more volatile components from the solids of the cutting fluid. The vapors are separated and the dried material discharged for further disposal. As with the aforementioned system embodied in U.S. Pat. No. 4,313,785, the system disclosed in U.S. Pat. No. 4,606,283 requires an excessive amount of energy input in order to accomplish separation of hydrocarbons from cutting fluids. The process does not provide optimum separation of volatile components under conditions which prevent the release of contaminated vapors to the surrounding environment and does not provide a system which is economically feasible especially in areas where low levels of hydrocarbon contamination are encountered.